United Nations Secretary-General Ban Ki-moon said late Wednesday he isn’t underestimating the gravity of the situation in Syria but believes there is an opportunity for progress and recommended the Security Council approve a 300-strong U.N. observer mission.

Mr. Ban said in a letter to the Council obtained by The Associated Press that he will consider developments on the ground, including consolidation of the ceasefire, before deciding on when to deploy the expanded mission, which is larger than the 250 observers initially envisioned.

The U.N. chief said the level of violence dropped markedly on April 12, the day a ceasefire called for by international envoy Kofi Annan went into effect, but that violent incidents and reported casualties have escalated again in recent days and “the cessation of armed violence in all its forms is therefore clearly incomplete.”

At the same time, Mr. Ban said, the Syrian government and opposition have continued to express their commitment to a ceasefire and have agreed to cooperate with a U.N. observer mission.

“I remain deeply concerned about the gravity of the situation in the country,” he said. “However, without underestimating the serious challenges ahead, an opportunity for progress may now exist, on which we need to build.”

Mr. Ban said Syria has not fully implemented its initial requirement under Mr. Annan’s six-point plan to withdraw troops and heavy weapons from towns and cities and return them to barracks.

On Wednesday, he said, the advance team visited Jobar, Zamalka and Arbeen in suburban Damascus and reported the presence of military at checkpoints and around some public squares and buildings in all three locations. In Arbeen, he said, one armored personnel carrier was hidden, covered by a plastic sheet.

“The situation in Arbeen became tense when a crowd that was part of an opposition demonstration forced United Nations vehicles to a checkpoint,” Mr. Ban said. “Subsequently, the crowd was dispersed by firing projectiles. Those responsible for the firing could not be ascertained by the United Nations military observers.”

The Secretary-General said no injuries were observed by the advance team but one U.N. vehicle “was damaged slightly during the incident.”

Mr. Ban said the team’s initial request to visit Homs the city at the centre of the 13-month conflict “was not granted, with officials claiming security concerns.”

The U.N. chief said action on other parts of Mr. Annan’s six-point plan “remains partial, and, while difficult to assess, it does not amount yet to the clear signal expected from Syrian authorities.”

Regarding the right to protest freely, he said, reports from local opposition groups suggest there was “a more restrained response” to demonstrations on April 13 the day after the ceasefire took effect “but there were nevertheless attempts to intimidate protesters, including reports of incidents of rifle fire by government troops.”

On detainees, Mr. Ban said “the status and circumstances of thousands of detainees across the country remains unclear and there continue to be concerning reports of significant abuses.” He added that “there has been no significant release of detainees.”

While the Syrian government said entry visas were granted to 53 Arab and foreign journalists, Mr. Ban said the U.N. has no further information and he again demanded that all journalists “have full freedom of movement throughout the country.”

Mr. Annan’s plan calls for unrestricted humanitarian access but Mr. Ban said “no substantive progress has been achieved over the last weeks of negotiations” on access to the one million people in need of aid.

“Developments since April 12 underline the importance of sending a clear message to the authorities that a cessation of armed violence must be respected in full, and that action is needed on all aspects of the six-point plan,” Mr. Ban said.

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